The stock market has been acting crazy recently, mostly in a bad way. Should investors be worried, and why? When the S&P 500 fell apart from 2000 to 2002, in hindsight it was easy to see what had happened: due to Y2K and some easy money, investors put unrealistic growth assumptions on so-called “new economy” stocks. When everything went south in 2008, we knew it was because of problem loans and debt securities made out...